COPD is now the 3rd leading cause of death worldwide. Age is the major risk factor for COPD, especially a major sub-type of COPD - emphysema (loss of functional airspaces). The cumulative effect of cigarette smoke exposure and environmental toxins across the lifespan are important contributing factors. Yet very little is known about the specific mechanisms and curative therapies do not exist. Our plans to define the mechanisms by which an innate immune receptor, Toll-like receptor 4 (TLR4), exerts anti-aging and protective repair responses during aging and cigarette smoke exposure will elucidate current gaps in the field and provide new therapeutic targets. We have established clinically relevant experimental models in which aged wild type mice develop emphysema-like airspace enlargement, which is accelerated by cigarette smoke exposure (CS). We also identified genetic mouse models of toll-like receptor deficiency (TLR4-KO) which develop premature emphysema in the absence of CS. Notably, we discovered that many of the molecular changes induced by TLR4 deficiency in mouse lung were recapitulated in the human COPD lung. The molecular basis for emphysema in aged wild type, TLR4-KO mice and human COPD appears to be the induction of senescence molecules, specifically increased p16, and epigenetics change involving Dnmt3a and Tet2. We also show that restoring endothelial TLR4 prevents emphysema in TLR4-KO mice and has similar anti-aging effects as calorie restriction. These results reveal a unifying paradigm in which decreases in specific innate immune pathways predispose to premature lung aging. Our overall hypothesis is that deficient endothelial TLR4 signaling leads to premature lung aging via increased p16 and decreased Dnmt3a and Tet2 expression. We will use aged mice, with and without CS, human samples and genetic approaches to test this hypothesis in the following Aims: 1) Determine the mechanisms of how TLR4 deficiency occurs with aging and CS in lungs and cells. 2) Determine the mechanisms of TLR4 deficiency-induced p16 expression in lungs and cells. 3) Determine the mechanisms of TLR4 deficiency-induced Dnmt3a and Tet2 suppression and their roles in p16 induction in lungs and cells. Our studies will expand our basic understanding of the molecular and cellular aspects of lung aging and inform future preventative or therapeutic approaches to age-related lung diseases, such as emphysema/COPD.